1. Field of the Invention
This invention relates to communications equipment, and more particularly to a method and apparatus for reducing the power consumption and increasing the frequency efficiency of a mobile communications unit used in a cellular communications network.
2. Description of Related Art
It has long been a goal of communications engineers to establish a mobile communication network that would allow an individual to maintain wireless communications with others. That goal is being realized today by a mobile cellular communication system, commonly referred to as Advanced Mobile Phone Service (AMPS), in which an area is geographically divided into cells. In addition to AMPS, a Cellular Digital Packet Data (CDPD) system has been implemented to allow wireless communication of digital data. A typical cell in a cellular system may be sectored or omni-directional. In a sectored cell, the coverage may be divided among several antennae that serve different regions of the cell. Typically, three to six sectors are used within a sectored cell. A Base Station associated with each cell sector controls airlink access to Mobile Units (which are typically mobile/cellular phones).
FIG. 1 illustrates a number of sectored cells 102 arranged to cover a relatively large geographic area. FIG. 2 illustrates a single sectored cell 102. Each cell 102 typically has three sectors 103, each of which is serviced by a corresponding one Base Station 101a, 101b, 101c having an independent antenna. Each sector 103 has a "footprint" 105 (i.e., an area that is within the range of the Base Station 101 for both transmit and receive signals), which may differ in size and shape from sector to sector. In accordance with one implementation of a cellular communications system, cell boundaries are preferably defined as a set of connected points, each of which have equal received power as observed at a Mobile Unit 109. The broken line 105 shown in FIG. 2 represents footprint boundaries. Typically, a number of cells 102 are arranged in proximity to one another, such that the sectors 103 of adjacent cells 102 overlap. Generally, such overlapping of sectors 103 of adjacent cells 102 ensures that a Mobile Unit 109 may maintain contact with at least one Base Station 101 from any location.
A Mobile Data Intermediate System (MDIS) 112 must know which Mobile Units 109 are within each cell so that communications to the Mobile Unit 109 may be properly routed. In addition, the Mobile Unit 109 must know on which forward and reverse channel to receive and transmit. In accordance with one cellular system, the particular forward and reverse channels on which the Mobile Unit 109 is to receive and transmit depend upon the relative signal quality of the signals received by the Mobile Unit 109 over the available forward channels. If the Mobile Unit 109 leaves a cell and attempts to transmit on the forward channel associated with that cell, there is an increased chance that the transmission will interfere with the transmissions of other Mobile Units 109 attempting to communicate with other Base Stations on the same reverse channel. This is likely because the same frequencies are allocated to more than one Base Station, as shown in FIG. 3. These Base Stations are separated by a distance which is sufficient to prevent interference with one another as long as each Mobile Unit 109 transmits that frequency only while within the boundaries of that cell.
As the Mobile Unit 109 moves away from the Base Station to which it is transmitting and from which it is receiving, the quality of the signal it receives will generally decrease. Concurrently, the quality of the signal present on the forward channel of the neighboring Base Station toward which the Mobile Unit 109 is approaching will generally improve. In accordance with the CDPD system, when the signal quality of the neighbor Base Station becomes greater than the signal quality of the selected Base Station, the mobile Unit 109 assumes that it has entered a new cell. At that point, the Mobile Unit 109 executes a cell transfer to the Base Station within the neighbor cell.
The need to transmit a message informing the MDIS 112 that the cell transfer has been executed and to monitor signal quality to determine when to request a cell transfer is typically not problematic when the Mobile Unit 109 consumes a relatively large amount of power during normal operation. For example, cellular telephones are required to transmit frequently and/or for relatively long periods of time. Therefore, the amount of power consumed by monitoring and transmitting cell transfer requests is not typically a significant portion of the power drain on the battery. However, other types of Mobile Units 109 are expected to consume less power and have longer battery life. For example, pagers typically are expected to have a battery life that extends for as much as one month or more. Repeatedly monitoring the forward channels and transmitting a message informing the MDIS 112 that a cell transfer has occurred each time a cell boundary is crossed requires a substantial amount of power. That is, monitoring each forward channel requires the Mobile Unit 109 to first receive each forward channel for a period of time. The signal quality of each channel must then be compared to determine which forward channel has the best signal quality. In systems which have the ability to limit power consumption by entering a dormant state or "sleep-mode", such monitoring requires the Mobile Unit 109 to awake in order to perform the monitoring function. Transmitting a cell transfer message is typically performed far less frequently than monitoring, but requires a relatively large amount of power.
Therefore, because of the power requirements associated with a cellular system (i.e., the need to monitor forward channels and perform cell transfers) and the desire to provide pagers with extended battery life, service providers are not currently using cellular communication techniques in paging systems. Rather, paging is typically performed today by broadcasting a page through each of the Base Stations within a geographic area using the same transmission frequency at each Base Station, eliminating the need for monitoring forward channels and performing cell transfers. This broadcast is commonly referred to as a "simulcast". The geographic area is determined by the nature of the service for which the client has subscribed. For example, when a paging service client orders paging service from a particular service provider, the client orders that service for a particular geographic service area, such as Southern California. Pages for that client are then transmitted on every Base Station in the geographic service area in which the client has paid for service.
Each client is provided with a mobile receiver and assigned a unique client number. The client number allows each client to be uniquely identified, and thus contacted. In a simulcast system, when a page for that client is received by the service provider, the service provider does not know where that client's mobile receiver will be. Accordingly, the client is paged by simulcasting a signal through each of the Base Stations in the entire service area. The signal that is transmitted indicates that there is a message pending for that client by transmitting the unique client number. In many paging systems in use today, pagers topically need only receive messages. However, some systems in use today for paging send an acknowledgment from the pager upon receipt of a message.
Paging systems may also reduce the power requirements of the Mobile Unit 109 by having each Base Station transmits a complete list of the client numbers for those clients for whom a page is pending on a regular basis. If the client's pager device does not see its client number in the list, it can revert to sleep mode until the next list is due to arrive. If the pager detects its client number, then the pager continues to receive transmissions from the Base Station in an attempt to receive the message intended for that client. After the message intended for that client is received, the Mobile Unit 109 ceases monitoring the forward channel except at regular intervals during which the list of clients is again transmitted. Due to the relatively limited bandwidth assigned to each service provider (typically 50 kHz), the use of such a simulcast paging scheme limits the number of clients that can be effectively serviced without extensive delays in the time required to contact the client. It can be seen that far greater capacity and lower delays are possible in a cellular communication system, since in a cellular communication system forward transmission to a Mobile Unit 109 is routed through only that particular Base Station associated with the Mobile Unit 109. However, these advantages are currently being sacrificed in order to reduce the power requirements of the Mobile Unit 109.
Accordingly, it would be desirable to provide a cellular communication system in which the power requirements of the Mobile Unit arc reduced so that the advantages of cellular communications may be realized without having to sacrifice bandwidth and/or battery life. It would also be desirable to provide a method and apparatus which allows more efficient use of available bandwidth in a paging scheme, but which does not need to transmit frequently for control and system maintenance, and thus uses power relatively efficiently.